The objective of this proposal is to clarify the mechanisms that underlie reserves of O2 transport in heart and red skeletal muscle. To achieve that goal the principal site of resistance to O2 mass transport must be identified. We shall use a cryospectrophotometer unique to this laboratory; its spatial resolution is -40 mum3. It has already been used to map gradients in myoglobin (Mb) saturation in individual myocytes. Proposed modifications of the instrument should allow measurement of hemoglobin (Hb) saturation in individual red cells, and Mb saturation in the pericapillary region close to the sarcolemma. The delta PO2 between Hb and pericapillary myoplasm defines the driving force for O2 release from blood. By combining theory with precisely localized measurements of Mb saturation the O2 flux from an individual capillary can be determined. The relationship between the driving force and the flux defines the transcapillary conductance for O2. This conductance is an unambiguous criterion with which to determine whether the capillary is the main resistance site, as recent models predict. Steep calculated transcapillary O2 gradients are due to the particulate nature of blood, absence of a heme protein O2 carrier between red cell and myocyte, and facilitated diffusion of O2 by Mb. We will evaluate these three factors by determining the driving force and transcapillary O2 conductance in working muscle under the following conditions: 1) O2 flux per red cell stressed by hypoxemia or anemia. 2) Exchange transfusion with blood of P50 0.5 or 1.5 x normal. 3) Increase in carrier-free diffusion path produced by muscle edema. 4) Exchange transfusion with a solution of stroma-free Hb of appropriate P50. 5) Decreased capillary surface area and short red cell path lengths and transit times produced by experimental cardiac hypertrophy. Results will clarify mechanisms of blood-tissue O2 transport in helth and in common clinical situations. The same experiments will provide insight into the clinical risks and benefits of administering or withholding blood or blood substitutes. Transport reserves must accommodate spatial and temporal heterogeneities of red cell flux. Maps of Hb and Mb saturation in multicellular arrays will be used to determine whether such heterogeneities are compensated by Mb-facilitated redistribution of O2 among myocytes, and interaction of capillary diffusion fields. Maps from muscles frozen under conditions 1-5 above, and during reflexly activated contraction of motor units, will define a functional unit of O2 supply. The anatomic scale of this unit will indicate the sphere of influence of microcirculatory heterogeneities, and the impact of small vessel disease in various clinical entities.